0:15Skip to 0 minutes and 15 secondsWe saw in the previous video that we're constantly called upon to use our logical and critical thinking skills. We suggested though that humans tend not to be very good at logical and critical thinking. In this and other steps this week, we'll give some illustrations of the ways in which we're rather easily led astray. These are common obstacles to effective, logical, and critical thinking. Here's an example of an obstacle to good critical thinking. "The human understanding, when it has once adopted an opinion, draws all things to support and agree with it.

0:49Skip to 0 minutes and 49 secondsAnd though there be a greater number and weight of instances to be found on the other side, yet, these it either neglects and despises or sets aside and rejects in order that by this great and pernicious predetermination, the authority of its former conclusion may remain inviolate." Francis Bacon is referring to what we now call the confirmation bias. So once again, Francis, but this time in plain English please. I mean that humans have a deep seated tendency to prefer information that confirms their existing positions. Once they've come to a view about something, they see the evidence that supports that view, and they overlook the evidence that doesn't. Is that plain enough? So how does confirmation bias work in real life?

1:36Skip to 1 minute and 36 secondsI'll give you a few examples. Let's start with a forensic case. We think of fingerprints as reliable and objective evidence. Well, following a notorious misidentification using fingerprint evidence, you'll see the article about that in the materials, researchers carried out a cunning experiment. They found five very experienced fingerprint experts who, half a decade earlier, had judged that fingerprints taken from a suspect matched those taken from a crime scene. This time though, they told them that the fingerprints were from the notorious misidentification case. And now, only one of them stuck with their earlier judgement. Three said there was definitely no match, and one said there wasn't enough information to decide. So what's going on here?

2:27Skip to 2 minutes and 27 secondsWhy did five very experienced experts change their minds? They fell victim to confirmation bias. When primed to think there was no match as part of the 2005 research, four of them saw the evidence that confirmed that judgement. That's Bacon's point. Here's a more everyday case of a kind I imagine we've all come across. Companies all over the world sell mattresses or mattress covers which contain magnets and which are said to cure all sorts of conditions. New Zealand has a version. Until recently, they had an advertisement, a radio jingle which went like this. (SINGING) How do you sleep at night with all that nagging pain? How do you sleep at night with all those aches and pains?

3:12Skip to 3 minutes and 12 secondsThe answer is Woolrest BioMag, because 400,000 Kiwis can't be wrong. After 17 years, our customers are our best endorsement. Yes, 17 years of giving drug free pain relief is well worth celebrating. What should we make of this? The company puts a lot of weight on testimonials, statements from satisfied customers. Knowing about confirmation bias might make you a bit suspicious of that. Why? Well, the people who give the testimonials have a hypothesis about the effectiveness of the mattress. They think it'll work; that's why they've spent $300 to $500 on it. They want it to work so they haven't wasted their money. Suppose, like most of us, they have good days and bad days.

3:58Skip to 3 minutes and 58 secondsSometimes, we're full of energy and leap out of bed, and other days, we feel a bit creaky and find it hard to get going. If you've bought one of these mattresses though, you notice the good days, and you attribute them to the magnets. That we'll see later in the course, when we talk about science, is why we do random controlled trials. Suppose you took the magnets out of half of the mattresses, but didn't tell people whether they had the magnetised or the non-magnetized version. Now, they wouldn't know how to interpret the evidence to confirm a preexisting opinion. It would be interesting to see if they gave the same testimonials.

Confirmation bias

Francis Bacon describes confirmation bias so:

“The human understanding, when it has once adopted an opinion draws all things … to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects and despises or … sets aside and rejects in order that by this great and pernicious predetermination, the authority of its former conclusions may remain inviolate”. (Francis Bacon 1602).